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Principles of Operation
Medium Attachment Unit
A functional block diagram of the HP 30241A MAD is shown in figure 3-4. As shown, the MAU has
fOUf functional areas:
Receive Function.
Receives serial data bit streams from the coaxial cable and sends these to the
LANIC.
Transmit Function.
Accepts serial data bit streams from the LANIC, and transmits this data to the
coaxial cable.
Collision Detection.
Detects the presence of two or more simultaneous transmissions on the coaxial
cable and informs the LANIC of this condition.
Jabber Fault Detection. Detects an abnormally long output data stream (lljabber") from the LANIC,
and inhibits transmission to the coaxial cable.
RECEIVER. The coaxial receiver has a high input impedance. (The high impedance is necessary to allow
up to 100 users to connect to the coax and not load down the cable.) The data received from the coax is
then amplified, shaped, and compensated for attenuation by the coaxial cable. The data is (hen sent
through a cable driver to the LANIC via the twisted pair of the AUI cable. The MAU also has a DC
squelch function which turns off the AUI cable driver when no data is present on the coax as indicated by
a lack of DC component in signals on the coaxial cable.
Since the receiver passes all data from the coaxial cable to the AUI cable, the receiver also monitors data
sent by its own LANIC, thus implementing a local loop-back and monitoring function.
TRANSl\UTTER. The transmitter accepts data from the LANIC and transnlits this data to the coaxial
cable. Data is transmitted to the cable using a high -impedance current sink. Current drive of the
coaxial cable causes multiple transmitters' signals to add without damage to the transmitters. The coaxial
cable driver adheres to the recommended values for drive level specified in the IEEE 802.3 Standard.
COLLISION DETECTION. When a collision occurs on the coaxial cable, the DC voltage on the cable
increases in magnitude and the collision detection circuitry senses this condition. The LANIC is informed
of this condition by a signal sent from the MAD on the CONTROL IN (CI) cable pair (part of the AUI
cable).
The collision detection scheme employed implements receiver-based collision detection.
This
means that the presence of two or more transmitters can be detected even when the MAU is not
transmit ting.
To insure that the collision detection circuitry is working satisfactorily, it is tested each time a data
packet is sent. At the end of each transmitted packet, a 'Heartbeat', or SQE (Signal Quality Error) test
signal (which is a short burst of the collision indicator signal) is sent to the LANIC.
Feb 85
3-18
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